Personally, in any situation, I only talk once. If the answer is ridiculous, I write a complaint letter. This is what I would do in your case: write an official complaint to them first that Motability prevents your grandfather from using the vehicle as intended and of course explain the situation in details, advising you will provide necessary medical information if requested.
Sent from a mobile device.
Apologies for briefness and spelling mistakes.
Motability Skoda Enyaq SportLine 85x April 2024 (unhappy customer - Ombudsman pending)
Motability Mazda CX-60 PHEV July 2023 (unhappy customer - early termination on mechanical grounds)
Motability VW Touran Family Pack May 2019 (happy customer)